Most modern automotive vehicles have one or more on board computers which control, for example, fuel injection and ignition. Each computer runs a self checking test which also checks the vehicle electrics. If a fault is found then a fault code signal is generated. Under normal circumstances, a mechanic would then have to look up this number in a vehicle manual which would provide him with various possible options for the type of fault. Because this tends to be a time consuming task mechanics tend not to do this and simply go through the vehicle changing various instrument sensors until they find the faulty sensor. However, more often than not, the fault lies in the wiring harness of the vehicle with a break in a cable.